Document text
STOP
Early Journal Content on JSTOR, Free to Anyone in the World
This article is one of nearly 500,000 scholarly works digitized and made freely available to everyone in
the world by JSTOR.
Known as the Early Journal Content, this set of works include research articles, news, letters, and other
writings published in more than 200 of the oldest leading academic journals. The works date from the
mid-seventeenth to the early twentieth centuries.
We encourage people to read and share the Early Journal Content openly and to tell others that this
resource exists. People may post this content online or redistribute in any way for non-commercial
purposes.
Read more about Early Journal Content at http://about.jstor.org/participate-jstor/individuals/early-
journal-content .
JSTOR is a digital library of academic journals, books, and primary source objects. JSTOR helps people
discover, use, and build upon a wide range of content through a powerful research and teaching
platform, and preserves this content for future generations. JSTOR is part of ITHAKA, a not-for-profit
organization that also includes Ithaka S+R and Portico. For more information about JSTOR, please
contact [email protected].
THE DESIGN AND OPERATION OF INTERMITTENTLY
OPERATED WATER PURIFICATION PLANTS
By N. T. Veatch
In our experience in the design and operation of water filtration
plants, we have learned that the plant with intermittent operation
presents the most difficult problems. The practice in the past has
been to depend almost entirely on the filters themselves, for the
overall efficiency of the plant, and to give very little attention to
the design of the settling and coagulating basins. There is little
question but what the action in the basins and in the filters them-
selves is equally important in getting good and economical results
in any filtration plant. This fact is most emphatically true of plants
which are operated intermittently.
In cases where the plant is operated perhaps ten hours each day,
it is difficult to get the proper conditions of water applied to the fil-
ters during the first part of each run, as the floe has almost always
settled out so as to cause the first water applied to the filters to be
clear, or practically so, and free from floe. The arranging of the
basins so as to make it possible to handle this clear water and to pre-
pare it for application on the filters is one of the most difficult
features in the design.
It is a pretty well established fact that thorough mixing of the
chemical and the raw water when first pumped into the basins is
well worth the cost. There is always some question as to just how
far it is wise to carry this mixing, as each water presents a separate
study in itself. There are so many things that enter into the action,
such as temperature, turbidity, etc., that no set length of time can
be taken as the proper time for mixing. The length of time also
depends on the degree of agitation. If the mixing is obtained by
means of mechanically driven agitators, the time required is much
less than when obtained by baffling. To meet different tempera-
tures, and different conditions of the water, it is probably best to
subject the water to at least thirty minutes of mixing when me-
chanically driven agitators are used, and at least an hour where
759
760 N. T. VEATCH
baffles are depended upon. These periods are based on the use of
alum as a coagulant, and are taken from results obtained at different
plants. If lime or lime and iron is used as coagulant, the length of
time for mixing would vary, depending upon the results desired,
such as to the degree to which it is desired to soften the water, if
softening is attempted. The figures given should be considered as
general only, and not be construed as the ones to use for any and all
waters.
After the water has had the chemical mixed with it thoroughly,
the question arises as to the best means of treating in the basins
before applying to the filters. This second stage in the treatment
should depend on the operation of the plant. If the operation is
continuous, we find that the water should be treated in the basins
in such a manner as to obtain the greatest amount of sedimentation,
and at the same time carry a proper floe over onto the filter beds.
In case the operation is intermittent, it is best to give the water the
greatest chance for sedimentation, and not attempt to keep any floe
in suspension. In this way the water at the start of any run, and
at any time during the run, is as near a completely settled water as
possible. This would also be the condition of a water which is sub-
ject to treatment in large settling basins, before applying to the
filters.
To prepare the flocless water obtained as outlined above, for ap-
plication to the filters, the best plan is to subject it to another thor-
ough mixing. It is well to arrange this secondary mixing so that
the chemical can be applied at several different points and periods
before the water reaches the filters. We have found that it is con-
venient to be able to apply coagulant at |, \, f and 1 hour periods,
just prior to water reaching the filters. The condition of the water,
and the temperature should determine the proper place for appli-
cation.
This paper is meant to deal particularly with plants where the
total retention period in the basins is from 4 to 6 hours. In basins
of this size it is possible to use the basins as settling and also coagu-
lating basins combined. As indicated, previously, in this paper, if
the operation is continuous, the mixing at the entrance to the basins,
and proper arrangement of the basins will put the water in good
condition for application. However, in intermittent operation ar-
rangements should invariably be made for applying the chemicals
just prior to passing the water onto the filters. The design of this
INTERMITTENTLY OPERATED PLANTS 761
secondary treating basin should be given very careful study, as the
chemicals should all be decomposed before reaching the niters.
The question may arise as to why it is necessary to design filter
plants which operate intermittently. This is a very natural ques-
tion, as it is unquestionably preferable to operate a filter plant con-
tinuously. However, the matter of cost, both first cost and that of
operation, must always be considered, and especially in the smaller
cities. It is necessary for the engineer to compromise his ideal in-
stallation to fit the appropriation, and give his client a plant which
will operate at a minimum cost. The average city of less than 3000
people does not operate its water works plant continuously, and al-
most invariably the entire operation is handled by one shift, and
often by one man. It generally proves to be much cheaper to pay
the fixed charge on the extra investment necessary to give the city a
plant that will fit in with the normal operation of the pumping plant,
than to cause the city to change its plan of operation, and add
one or two shifts to its operating force.
With a properly designed plant which operates intermittently,
the city has a plant which will fit in with its normal operation, and
as the city grows nearer the size which would require the maximum
capacity of the plant, it can well afford to put on the extra help
necessary to operate continuously.
It is evident that there is a great need for good engineering and
careful study in the design of basins, for any plant. Some of the
discredit which is given to mechanical filtration could have been
avoided if proper care had been given to preparing the water for
treatment on the filters.